Medium and high consistency pulp (consistency 8-30%) forms a very stiff material; it can be so stiff that one can stand with ordinary shoes on the pulp and not sink into the pulp. The reason for it is that fiber with the length of a few millimeters form a strong-three dimensional fiber network. The fibers are rather rigid and when they rest on each other they form a strong structure.
High consistency pulp is still generally pumped by a displacement pump or a screw pump. This was up till now the only way to pump high consistency pulp. When the pulp is pumped by such pumps, no control valve is used in the discharge side. There are two reasons for that. Firstly, high consistency pulp is stiff and the pumps produce a pulsating pressure. If there is a throttling in the discharge side, it produces strong pressure pulses in the pipe system, which can break structures. Secondly, a displacement pump works even though if there is no pulp in the suction side or so little pulp that it would only partly fill the compartments of the pump. The pump can work so that it pumps forward everything that comes from the suction side to the compartments.
High consistency pulp can, however, be changed momentarily into a flowing state by the breaking fiber network by bringing shear forces to the suspension. This is called fluidization of high consistency pulp. Normally fluidization is effected by some kind of powerful rotor. For example, in a high consistency pump the rotor effects the fluidization in the suction duct of the pump. Fluidization is a reversible process, and as soon as the rotor is stopped or the pulp is no longer in the range of the rotor, the fiber network is formed again and the suspension becomes again a substantially solid material.
Lately, a new type of high consistency pump has become accepted in the mills, cf., e.g. U.S. Pat. No. 4,435,122. This new kind of pump is a centrifugal pump, in which high consistency pulp is fluidized, in other words changed into a flowing state just before the pulp reaches the range of the pump impeller. With this technique in pumping high consistency pulp it is usually necessary to assemble a control valve in the discharge side of the pump according to FIG. 1 in order to ensure that there is always pulp in the suction side of the pump or that the amount of the pulp to be pumped is correct. Reason for this is the fact that a centrifugal pump always has to have its inlet opening filled with pulp and thus its operation is most often controlled in such a way that the pulp level in the vessel prior to the pump is maintained substantially constant by means of throttling the discharge of the pump.
If the capacity of a pump is adjusted by a control valve according to FIG. 1, the head of the pump is considerably lowered in the control valve because of the rigid character of the pulp. In controlling the flow of high consistency pulp the resistance i.e. pressure drop is often several bars even with the best valves known.
Often in pulp mills a pulp pump is followed by yet another device, namely a mixer for mixing chemicals into the pump, for example, in bleaching. The mixing may be effected either in a separate mixer or in a high consistency pump. When chemicals are mixed in a medium/high consistency pump, they are added either before the pump or at the outer rim of the impeller. If this is done, a separate mixer is not necessary, but the same device may serve both as a mixer and a pump. However, if the chemicals are in the form of gases they cannot be added before the pump as centrifugal pumps pumping medium or high consistency pulp are usually provided with means for separating gas from pulp whereby the chemicals would be the first gases to be separated.
Often a pump cannot be used as a mixer for several reasons. Such may be, for example, material problems or the fact that the amount or the quality of the chemicals in such that the chemicals cannot be added into the pump. Hereby one has to use a separate mixer according to FIG. 2. There are cases in which part of the chemicals may be fed to the pump and the rest to the mixer or all chemicals to the mixer depending on the situation.
A mixer being used in connection with medium or high consistency pulps consists of a substantially cylindrical casing with an axial inlet opening and an axial outlet opening and protrusions on the inner surface, of an axially rotating rotor which has protrusions on the outer surface, and of a feed duct for chemicals which opens to a substantially annular mixing zone between the rotor and the casing. Said mixer, although very practical and reliable, is, however, rather complicated to manufacture of special material. This kind of a mixer has still a severe drawback relating to its operation. After being efficiently mixed with the chemicals and having started to rotate in the mixing zone due to the rotation of the rotor the pulp leaves the annular mixing zone in a direction coaxial with the rotor whereby the rotational movement of the pulp continues and the pulp tends to form a rotating annulus on the wall of the flow channel. Also the pressure in the pulp decreases rapidly as the flow area of the pulp suddenly increases. The result of these two factors is that the gases, i.e. most often the chemicals, tend to separate in the middle of the flow starting to form gas bubbles. Thus the mixing of the chemicals with the pulp is not as efficient and even as could be expected.
As one can note from the foregoing there are several different devices utilized in connection with the pulp pump, namely the pump itself, the control valve and a chemical mixer. A purpose of the present invention is to combine these devices in such a way that the result would be most economical in the point of view of both the manufacturer of the devices and their users.
It would, of course, be possible to combine all of the above mentioned three devices together, but there is one factor which makes it practically impossible. As explained above, the chemicals may be mixed within the pump, wherefore the pump perhaps should be manufactured of a more durable material, but not always. However, when pumping medium or high consistency pulps the control valve should be arranged so close to the pump impeller that the fluidized pulp would not have time to reverse back into plug flow by forming a fiber network. It is to be noted at this stage that the discharge duct of a centrifugal pump is always tangential in order not to resist the flow out of the pump. On one hand, this kind of tangential arrangement would be impossible in case high consistency pulps were to be pumped, as the distance from the impeller blades maintaining the fluidized state at a sufficient level to the throttling arrangement arranged in a tangential discharge duct would be long enough for allowing the pulp to form fiber network, whereby an ordinary valve would either cause a high pressure loss or be entirely clogged. On the other hand, the discharge duct could be arranged substantially radially, whereby the above defined distance would be minimized, but the sharp turn in the volute flow from circumferential to radial would bring about a considerable drop in pressure substantially equalling the drop in an ordinary valve.